1. Field of the Invention
The present invention relates to the continuous and efficient production of low concentrations of aqueous chlorine dioxide from a dilute continuous feed stream of aqueous chlorite.
2. Description of the Related Art
Chlorine dioxide, ClO2, is a greenish yellow gas with an appreciable solubility in water. It is a powerful, yet selective, oxidant that can kill fungi, bacteria and algae in a variety of applications, among which are the treatment of drinking water, the processing of wastewater, the disinfection of fruit and vegetables, the processing of poultry, and the control of microbiological growth in cooling towers. The benefit of chlorine dioxide as compared to other oxidizing agents in this class, such as chlorine, is that chlorine dioxide's use does not generate unwanted and perhaps hazardous chlorinated organic byproducts. Chlorine dioxide is a powerful oxidizing agent that is readily soluble in water. In its pure gaseous state, it is unstable and cannot be compressed, shipped or stored. Accordingly, chlorine dioxide must be generated at the point of use.
Chlorine dioxide can be generated from a variety of chlorine-containing starting compounds including various salts of the oxychlorine anions such as chlorite and chlorate. The chlorite ion readily reacts with acids or chlorine to form ClO2. Ultraviolet irradiation of aqueous solutions containing the chlorite ion will also generate chlorine dioxide.
Due to the nature of chlorine dioxide, it must be created or generated at the point of consumption or use. Commercially viable technologies have limited its use to applications where generally quantities greater than 5 pounds per day of chlorine dioxide are required.
U.S. Pat. No. 4,414,180 and U.S. Pat. No. 4,456,511 disclose a method for generating chlorine dioxide gas by metering nitrogen or air directly into a generator containing aqueous sodium chlorite, and illuminating by one or more incandescent or fluorescent lamps. The sodium chlorite is photochemically oxidized to chlorine dioxide and the resulting chlorine dioxide is taken into the gas phase by the nitrogen or air gas stream.
U.S. Pat. No. 4,874,489 to Callerame (1989) discloses generating a batch of chlorine dioxide by subjecting chlorite to ultraviolet radiation. The chlorine dioxide can easily be displaced from the reaction with air and dissolved in water to form chlorous acid.
U.S. Pat. No. 6,171,558 discloses a chlorine dioxide generator for use with a container holding aqueous chlorite. The apparatus includes a lamp for generating radiation, a connector or cap attached to the lamp so that the lamp is positioned within the container, and a circulation tube proximate to the light for circulating the aqueous chlorite in contact with the lamp radiation to create chlorine dioxide. Finely divided gas bubbles sweep microbubbles of chlorine dioxide upwardly from the exterior surface of the quartz housing through exit apertures and thence to the field of use. After the aqueous chlorite solution is depleted, the ultraviolet portion of the generator can be removed and reinstalled in a new container of aqueous chlorite, or moved to another location.
For each of the above noted patents, (despite assertions in U.S. Pat. No. 6,171,558) the chlorine dioxide is produced in a batch process and a gas is used to sweep the chlorine dioxide as a gas from the generator vessel.
There are a number of shortcomings with the prior art devices and methods for producing chlorine dioxide by irradiating an aqueous chlorite:    1. Batch processes compared with continuous processes inherently require more equipment and higher cost.    2. Removing the chlorine dioxide as a gas significantly reduces the efficiency which can be obtained.    3. With a batch process, the pH typically increases above 10 resulting in lower conversion of chlorite to chlorine dioxide.    4. With a batch process, the heat from the lamp(s) is not dissipated resulting in a higher lamp operating temperature with a lower ultraviolet wattage output.    5. With a batch process, there exists significant photolytic decomposition of the product chlorine dioxide resulting in a low conversion of chlorite to chlorine dioxide.    6. With a batch process, there exists significant unwanted side reactions that produce unwanted by-products and reduce the conversion efficiency.